It certainly looks like aged RAF blue-grey but I've seen a 'dans son jus' ex-BEF 3SW with that shade underneath other colours too. The 3SW had an RASC census number and was wartime production. I wonder if there were quantities of ex-airforce paint in France in the late 1940s ?
Refinishers at that time certainly had access to caustic dips to completely remove paint finishes and owners were tired of khaki !
There is no suggestion of RAF contracts in the KM ledgers. The Norton books refer to 'Department of Army Contracts' or 'Air Ministry / RAF' but the BSA books, simply 'War Office' which should imply Army. The assembly books also list fuel tank colours so we can see 'Green' or 'Bronze Green' for army (British, Dutch and Irish), Yellow for AA and C/S (Chrome / Silver) for the civilian models. A quick look shows no blue or grey.
Are you able to see anything of the frame number ? Any clues at all?